The invention relates to a frequency-generator circuit containing a parametric frequency-divider.
Oscillators for use in phase-locked loops (PLLs) often require a frequency-divider section to reduce the input frequency to a value which can be handled by the phase detector. An example of a frequency-divider which operates on the parametric principle is disclosed in German patent DE 195 29 529 issued on Oct. 31, 1996 to Robert Bosch GmbH and is shown in FIG. 1. In FIG. 1 the central element is a varactor diode 10 which acts as a non-linear reactance. Upstream of the diode is an input filter 11, which is a high-pass filter tuned to the frequency of the input signal E and functions also as an impedance transformer matching the impedance of the line at the input of the circuit to the lower input impedance of the diode 10. Downstream of the diode is an output filter 12, which is a low-pass filter tuned to the divided-down frequency of the output signal A and also functions as an impedance transformer in the reverse sense to that of the input filter 11. A bias network for the diode 10 comprises an L-C circuit 13, via which a bias voltage Uv is fed to the diode. A resistance R provides damping so that energy is not absorbed by the L-C network and no undesired oscillations are generated therein.
The varactor 10 possesses a capacitance-voltage characteristic given by the expression:   C  =            C      0                      (                  1          +                      U            φ                          )            γ      
where U is the applied voltage, C0 is the capacitance at U=0, "PHgr" is the voltage on the inner diode contacts (ignoring the series loss-resistance) and xcex3 is an exponent governed by the physical characteristics of the individual diode. In order to ensure that the diode will operate in parametric mode without undue pump power being consumed, a diode is required having a high gamma (xcex3) over at least a part of its capacitance-voltage characteristic. FIG. 2 shows a typical gamma characteristic with variation in gamma plotted against applied voltage U. In this particular case the diode is biased so that it is operating in the region defined by the value Uv, i.e at the point at which gamma is maximum.
While this known circuit provides frequency-division at very low phase-noise, for example, it involves quite a lot of outlay considering it must also be used in conjunction with an oscillator having some form of additional frequency-adjusting facility.
In accordance with the present invention there is provided a frequency-generator circuit as recited in claim 1. Advantageous realisations of the circuit are set forth in the subclaims.